Double-cascode two-stage operational amplifier

ABSTRACT

A two-stage op-amp circuit including a double-cascode telescopic op-amp circuit in the input stage and a fully-differential op-amp circuit in the output stage and having very high open-loop DC gain, very high unity-gain frequency, and relatively very low power consumption is presented. The input stage op-amp circuit and the output stage op-amp circuit are each comprised of a plurality of electrically connected MOSFET&#39;s. The input stage op-amp circuit provides very high gain, high input resistance, and large common mode rejection. The output stage op-amp circuit provides gain, low output resistance, and minimal output loss.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims the benefit of co-pending U.S.provisional patent application, issued Serial No. 60/224,601, and filedAug. 11, 2000, which is hereby incorporated by reference in itsentirety.

FIELD OF THE INVENTION

The present invention generally relates to integrated circuit (“IC”)operational amplifiers. More specifically, the invention relates tocascode, two-stage operational amplifiers.

BACKGROUND OF THE INVENTION

Since the development of the transistor and integrated circuits, ICoperational amplifiers (“op-amps”) have been used in many applicationsrelated to such fields as telecommunications, audio/video technology,and computers. Complementary metal oxide semiconductor (“CMOS”)operational amplifiers are a major category of IC op-amps that are usedin a variety of applications. For example, CMOS op-amps are widelyapplied in high performance CMOS circuits, such as high speed/highresolution pipeline analog-to-digital (“A/D”) converters.

In the application of op-amps, such as CMOS op-amps, the performance ofthe overall circuit is typically limited by the performance capabilityof the op-amp(s) used in the circuit. For example, the dynamic rangeperformance and frequency response performance of a circuit using anop-amp are generally limited based on the open-loop DC gain andunity-gain frequency of the op-amp. High performance circuits requireop-amps with high open-loop DC gain and a high unity-gain frequency.

There have been continuous efforts made to develop op-amps with higheropen-loop DC gain and a higher unity-gain frequency to support thedevelopment of increasingly higher performing circuits. Typically, inthis regard, there are prominent trade-offs that occur in op-ampperformance. For example, an increase in gain performance typicallyresults in an undesired decrease in frequency response performance. Asanother example, an increase in speed/unity-gain frequency typicallyresults in the undesired increase of power consumption.

Various op-amp designs have been developed in an attempt to meet highperformance requirements while minimizing performance trade-offs. Thesedesigns include folded-cascode op-amps with gain boosting and two-stageop-amps. Designs denoted by these names are known in the art, and neednot be described herein. Yet, such designs have not been shown to fulfilthe increasing demand for even higher performance applications. Thus,there is a need for an operational amplifier that performs with veryhigh open-loop DC gain while maintaining a very high unity-gainfrequency, in comparison to existing designs, but without significantincrease of undesired parameters.

SUMMARY OF THE INVENTION

Certain objects, advantages, and novel features of the invention will beset forth in part in the description that follows and in part willbecome apparent to those skilled in the art upon examination of thefollowing or may be learned with the practice of the invention. Theobjects and advantages of the invention may be realized and obtained bymeans of the instrumentalities and combinations particularly pointed outin the appended claims.

To achieve various objects and advantages, the present invention isdirected to a novel op-amp circuit. In accordance with a preferredembodiment of the present invention, a two-stage op-amp circuitincluding a double-cascode telescopic op-amp circuit in the input stageand a fully-differential op-amp circuit in the output stage is provided.

One advantage of a preferred embodiment of the present invention is thatit performs with a very high open-loop DC gain in comparison to existingop-amp circuits. Another advantage of a preferred embodiment of thepresent invention is that its significant increase in open-loop DC gaindoes not result in a decrease in its unity-gain frequency, as typicallyoccurs in existing op-amp circuits. That is, a preferred embodiment ofthe present invention performs with a very high open-loop DC gain whilealso maintaining a very high unity-gain frequency. Yet another advantageof a preferred embodiment of the present invention is that, consideringits very high open-loop DC gain and very high unity-gain frequencyperformance, it has no increase in power consumption and a significantlysmall increase in other undesirable operation parameters in comparisonto existing op-amp circuits.

Other objects, features, and advantages of the present invention willbecome apparent to one skilled in the art upon examination of thefollowing drawings and detailed description. It is intended that allsuch additional objects, features, and advantages be included hereinwithin the scope of the present invention, as defined by the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more fully understood from the detaileddescription given below and from the accompanying drawings of apreferred embodiment of the invention, which however, should not betaken to limit the invention to the specific embodiments enumerated, butare for explanation and for better understanding only. Furthermore, thedrawings are not necessarily to scale, emphasis instead being placedupon clearly illustrating the principles of the invention. Finally, likereference numerals in the figures designate corresponding partsthroughout the several drawings.

FIG. 1 is a circuit diagram illustrating a conventional two-stage op-ampusing a telescopic op-amp circuit in the input stage and afully-differential op-amp circuit in the output stage, as is known inthe prior art.

FIG. 2 is a circuit diagram of a two-stage op-amp using a double-cascodetelescopic op-amp circuit in the input stage and a fully-differentialop-amp circuit in the output stage, in accordance with a preferredembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Having summarized the invention above, reference is now made in detailto the description of the invention as illustrated in the drawings.While the invention will be described in connection with these drawings,there is no intent to limit it to the embodiment or embodimentsdisclosed therein. On the contrary, the intent is to cover allalternatives, modifications, and equivalents included within the spiritand scope of the invention as defined by the appended claims. Indeed,the present invention is believed to be applicable to a variety ofsystems, devices, and technologies.

Turning now to the drawings, wherein like referenced numerals designatecorresponding parts throughout the drawings, FIG. 1 shows a conventionaltwo-stage op-amp circuit 100, as is known in the prior art. Such acircuit includes a positive (i.e., noninverting) input 106 and anegative (i.e., inverting) input 108 electrically connected to the firststage 102. A positive (i.e., noninverting) output 110 and a negative(i.e., inverting) output 112 are electrically connected to the secondstage 104 of such a circuit and a load capacitor (165, 166) iselectrically connected to each of these outputs.

The first stage 102 of the op-amp 100 consists of what is commonlyreferred to in the art as a telescopic op-amp configuration, which ismade up of a plurality of metal-oxide semiconductor field-effecttransistors (“MOSFET's”) (124-132) that are electrically connected. Thefirst stage 102 functions to provide gain, that is to amplify the inputsignal, and also to provide high input resistance and large common moderejection. The second stage 104 of the op-amp 100 consist of what iscommonly referred to in the art as a fully-differential op-ampconfiguration, which is made up of a plurality of MOSFET's (133-137)that are electrically connected, as shown. The second stage 104functions to also provide gain and to provide a low output resistanceand minimal output signal loss. Electrically interconnecting the firststage 102 and the second stage 104 are coupling resistors (161, 164) andcoupling capacitors (162, 163).

The op-amp 100 is powered from a positive voltage power rail 120 and anegative voltage power rail 122. Additionally, various members (124-129,132-134, 137) of the plurality of MOSFET's, which make up the two stages(102, 104) of the op-amp circuit 100, are biased via bias inputs(114-117). Essentially, the conventional op-amp circuit 100 operates tooutput an amplified version of an input signal that has a frequencywithin the operating bandwidth of the op-amp 100. The op-amp 100utilizes Miller compensation to maintain stability, as is known in theart.

Having described a conventional two-stage op-amp circuit, as is known inthe prior art, reference now will be made to various embodiments of afully-differential two-stage op-amp with gain-boosting in the firststage, constructed in accordance with various aspects of the presentinvention. To this end, reference is first made to FIG. 2, which shows acircuit diagram of a two-stage op-amp 200 using a double-cascodetelescopic op-amp in the input stage and a fully-differential op-ampcircuit in the output stage, in accordance with a preferred embodimentof the present invention. Where appropriate, reference numerals havebeen kept the same as those illustrated in connection with FIG. 1 toemphasize the differences in a preferred embodiment of the presentinvention from the prior art circuit.

Referring to FIG. 2, the op-amp 200 of the present invention comprises apositive input 106 and a negative input 108 which are electricallyconnected to an input stage 202. The op-amp 200 also comprises apositive output 110 and a negative output 112 which are electricallyconnected to an output stage 204. In a preferred embodiment, as shown inFIG. 2, each output (110, 112) is also electrically connected to a loadcapacitor (165, 166), although in other embodiments, these loadcapacitors may not be included. An input or output, as it is referred toin the present invention, may include but is not limited to any thingthat may be used as an electrical connector or connection, for example,a terminal, pin, contact, lead, or solder point. The input stage 202 andoutput stage 204 are electrically coupled, and in a preferredembodiment, as shown in FIG. 2, the input stage 202 and the output stage204 are electrically interconnected by circuits that each have acoupling resistor (161, 164) and a coupling capacitor (162, 163).Additionally, the op-amp 200 is powered from a positive voltage powerrail 120 and a negative voltage power rail 122.

The input stage 202 comprises a plurality of MOSFET's (124-132, 138-141)that are electrically connected in a double-cascode telescopic op-ampconfiguration. The double-cascode configuration and the telescopicconfiguration are commonly referred to op-amp configurations in the art.The output stage 204 comprises a plurality of MOSFET's (133-137) thatare electrically connected in what is commonly referred to in the art asa fully-differential op-amp configuration. The op-amp 200 has members(124-129, 132-34, 137-141) of the plurality of MOSFET's of the input andoutput stages (202, 204) that are electrically connected to and biasedthrough bias inputs (114-119). In contrast to the conventional op-amp100 (FIG. 1), the op-amp 200 of the present invention (FIG. 2) includesthe additional MOSFET's (138-141) that transform the basic telescopicconfiguration of the conventional op-amp 100 to the double-cascodetelescopic configuration of the present invention. Further, in contrastto the conventional op-amp 100 (FIG. 1), the additional MOSFET's(138-141) of the op-amp 200 (FIG. 2) of the present invention are biasedthrough the additional bias inputs (118-119).

In the present invention, the input stage 202 functions to provide veryhigh gain and also to provide high input resistance and large commonmode rejection. Furthermore, the output stage 204 of the presentinvention functions to provide additional gain and to provide a lowoutput resistance and minimal output signal loss.

As a result of the double-cascode telescopic configuration in the inputstage of the op-amp 200, several notable benefits, among others, areachieved over the conventional op-amp 100. First, the open-loop DC gainof the op-amp 200 (FIG. 2) is significantly increased over that of theconventional op-amp 100 (FIG. 1). Second, the op-amp 200 achieves thissignificant increase in open-loop DC gain without a resultant decreasein its unity-gain frequency, unlike in the conventional op-amp 100 whichtypically incurs a decrease in unity-gain frequency for an attemptedincrease in open-loop DC gain. Third, the op-amp 200 achieves very highopen-loop DC gain and maintains very high unity-gain frequency with noadditional power consumption in comparison to the conventional op-amp100. Additionally, the op-amp 200 achieves these benefits while avoidingthe complex stability issue of the pole-zero doublet that occurs withthe use of active gain-boosting circuits in the op-amp design.

It is emphasized that the above-described embodiments of the presentinvention, particularly any “preferred” embodiments, are merely possibleexamples of the implementations that are merely set forth for a clearunderstanding of the principles of the invention. It will be apparent tothose skilled in the art that many modifications and variations may bemade to the above-disclosed embodiments of the present invention withoutdeparting substantially from the spirit and principles of the invention.All such modifications and variations are intended to be included hereinwithin the scope of the disclosure and present invention and protectedby the following claims.

Therefore, having thus described the invention, at least the followingis claimed:
 1. An operational amplifier comprising: an input stagecomprising a first plurality of metal-oxide semiconductor field-effecttransistors electrically connected in a double-cascode telescopicoperational amplifier configuration; a positive input electricallyconnected to the input stage; a negative input electrically connected tothe input stage; a first series circuit comprising a first couplingresistor and a first coupling capacitor electrically connected to theinput stage; a second series circuit comprising a second couplingresistor and a second coupling capacitor electrically connected to theinput stage; an output stage electrically connected to the first seriescircuit and to the second series circuit comprising a second pluralityof metal-oxide semiconductor field-effect transistors electricallyconnected in a fully-differential operational amplifier configuration; apositive output electrically connected to the output stage, wherein afirst load capacitor is electrically connected to the positive output; anegative output electrically connected to the output stage, wherein asecond load capacitor is electrically connected to the negative output;a first plurality of bias inputs electrically connected to the inputstage; and a second plurality of bias inputs electrically connected tothe input and output stages.
 2. An operational amplifier comprising: aninput stage comprising a first plurality of metal-oxide semiconductorfield-effect transistors electrically connected in a double-cascodetelescopic operational amplifier configuration; an output stageelectrically coupled to the input stage and comprising a secondplurality of metal-oxide semiconductor field-effect transistorselectrically connected in a fully-differential operational amplifierconfiguration.
 3. The operational amplifier of claim 2 wherein theoutput stage is electrically coupled to the input stage by a firstcircuit comprising a first coupling resistor and a first couplingcapacitor and by a second circuit comprising a second coupling resistorand a second coupling capacitor.
 4. The operational amplifier of claim 2wherein a positive input and a negative input are electrically connectedto the input stage, a positive output having a first load capacitor anda negative output having a second load capacitor are electricallyconnected to the output stage, a first plurality of bias inputs areelectrically connected to the input stage, and a second plurality ofbias inputs are electrically connected to the input stage and the outputstage.
 5. A method for providing an operational amplifier having a veryhigh open-loop DC gain, a very high unity-gain frequency, and arelatively very low power consumption comprising the steps of: providingan input stage having very high gain, high input resistance, and largecommon mode rejection and comprising a first plurality of metal-oxidesemiconductor field-effect transistors configured in a double-cascodetelescopic operational amplifier configuration; and providing an outputstage electrically coupled to the input stage having gain, low outputresistance, and a minimal output loss characteristic and comprising asecond plurality of metal-oxide semiconductor field-effect transistorsconfigured in a fully-differential operational amplifier configuration.6. The method of claim 5, further comprising the step of coupling theoutput stage to the input stage through a first circuit comprising afirst coupling resistor and a first coupling capacitor and through asecond circuit comprising a second coupling resistor and a secondcoupling capacitor.